1. Field of the Invention
The present invention relates to a toner composition for electrophotography. More specifically, the present invention relates to a toner composition for electrophotography which is excellent in fixing ability, resistance to offset and image-forming properties.
2. Description of the Prior Art
In the field of the electrophotography, there have recently been developed a variety of copying apparatuses which make use of various fixing processes and a fixing process which has recently been most commonly used is the heat fusing-roll process. It has recently been required to greatly increase copying speed along with handling an increase in the quantity of information to be processed. In a high speed copying apparatus whose process speed exceeds 340 mm/s, the heat of the heat fusing-roll is transferred to copying paper in only a small quantity per unit area of the copying paper, while the paper takes a large quantity of heat away from the heat fusing-roll which leads to an increase in the heat loss. For this reason, the surface temperature of the heat fusing-roll is markedly decreased.
On the other hand, as household copying apparatuses and printers of electrophotographic systems are widely used, the size of the copying apparatus has been made more and more compact and the temperature as well as pressure of the heat fusing-roll has gradually been lowered or reduced.
As a result of the foregoing speeding up and miniaturization of the copying apparatuses, the conventional toners cannot be made sufficiently molten by the heat from the heat fusing-roll and, therefore, the conventional toners suffer from a problem such that they exhibit only insufficient fixing ability.
It may be thought that the foregoing problem can be solved by reducing the molecular weight of a resin used as a binder and hence the viscosity of the resulting toner in its molten state to thus improve the flow properties and hence the fixing ability of the toner. However, this becomes a cause of another problem that the toner tends to adhere to the surface of the heat fusing-roll and the adhered toner is again transfered to the subsequent copying paper (so-called offset phenomenon). Moreover, the strength of the binder is lowered because of the low molecular weight of the resin used. More specifically, images thermally fixed on the paper are damaged through bending and rubbing of the paper and as a result, the toner adhered to the paper drops off to thus cause defects of the images. Therefore, this method does not provide a toner having excellent fixing ability.
Under such circumstances, there has long been desired for the development of a toner composition which can be fixed at a low temperature, has high resistance to mechanical breakage and does not cause any offset phenomenon.
For instance, Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") No. Sho 55-6895 discloses a method for preparing a toner composition as a developer having good resistance to offset. The method employs, as a binder, a resin having a ratio of weight-average molecular weight to number-average molecular weight ranging from 3.5 to 40 and a number-average molecular weight ranging from 2,000 to 30,000. Further, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 49-101031 discloses a method for preparing a toner composition which does not cause offset phenomenon at all even at a relatively high fixing temperature in which a crosslinked resin is used as a binder to thus broaden the range of fixing temperature of the resulting toner.
However, these methods do not achieve a sufficient effect of inhibiting the offset phenomenon in high speed and small-sized copying apparatuses in which it is hard to transfer sufficient heat from the heat fusing-rollers to copying paper. In other words, if resins having a high weight-average molecular weight or those crosslinked are used as binders in order to prevent the offset phenomenon, the fixing ability of the resulting toner is impaired because of the high viscosity of the resin. On the contrary, if resins having a low weight-average molecular weight or those free of crosslinks are used as binders and hence the viscosity of the resin used is reduced, the images thermally fixed onto paper are broken upon bending or rubbing the paper and as a result, the toner adhered to the paper drops off to thus cause defects of the images and the offset phenomenon. Thus, the quality of images would be substantially impaired.
As a further means for solving the foregoing problems, J.P. KOKAI No. Sho 48-75033 proposes pressure-sensitive deformable pressure-fixable toner which comprises, for instance, a block copolymer comprising a combination of a hard resin and a soft resin. However, when such a block copolymer is used alone, the offset phenomenon arises and thus it cannot be used as a toner for heat fusing. In addition, the pulverization of the block copolymer is substantially difficult if it is pulverized by the existing conventional kneading and pulverization method. This results in an extreme reduction in yield of toners.
Further, J.P. KOKAI Nos. Sho 55-88071, Sho 55-88073 and Sho 55-90958 disclose magnetic toners excellent in resistance to offset and fixing ability which are obtained by melting and kneading a mixture containing, for instance, a styrene resin, a ternary block copolymer of styrene-butadiene-styrene, a releasing agent such as polybutene, polybutadiene, chlorinated paraffin, polystyrene, polyethylene and polypropylene, magnetic powder and carbon black. However, it has been confirmed that it is difficult to uniformly disperse, in the styrene resin, the ternary block copolymer, the releasing agent and an agent for adjusting electrification through melting and kneading thereof for a short period of time and that contamination of a light-sensitive material is liable to cause due to filming of the light-sensitive material. Moreover, when a large amount of the styrene-butadiene-styrene is used, the ingredient cannot be subjected to conventional pulverization, for instance, pulverization by a jet mill after kneading and must be pulverized while freezing or cooling the ingredients.